1. Field of the Invention
The present invention relates to a manufacturing method for a resin spring capable of generating a compressive dam ping force.
2. Description of Related Art
Resin springs are used at various places where absorption of vibration is required. For example, a compression spring formed of an elastomer described in Japanese Patent (PCT international application) Laid-open No. 7-502698 is produced by joining hollow noncircular bodies together to form a preform, and compressing the preform to orient its molecular structure for the purpose of removing a compressive setting.
However, when reducing the wall thickness of the preform to allow for a large displacement, increased creep occurs causing variations in characteristics. Therefore, sufficient energy absorption cannot be obtained.
To cope with this problem, the present Applicant has proposed a method of inserting inner-pressure generating members into the hollow portions of the resin spring (Japanese Patent Application No. 9-64714). 
By inserting the inner-pressure generating members into the hollow portions of the resin spring, sufficient energy absorption can be obtained with a large displacement, and the creep generated can be reduced.
In the above method, however, the preform is preliminarily compressed to be formed into a spring body having an anti-setting performance. The inner-pressure generating members are then inserted into the hollow portions of the spring body, thereby obtaining the resin spring. Accordingly, there are variations in permanent deformation of the spring body produced by the anti-setting treatment, so that the inner-pressure generating members inserted in the hollow portions of the spring body are not sufficiently in close contact with the spring body. As a result, there is a possibility that the spring body and the inner-pressure generating members inserted therein may slip relative to each other because of repeated deformation in using the resin spring, causing variations in load characteristics of the resin spring.
It is accordingly an object of the present invention to provide a manufacturing method for a resin spring which can improve the contact between the spring body and the inner-pressure generating members to thereby reduce variations in load characteristics due to repeated deformation in use.
According to a first aspect of the present invention, there is provided a manufacturing method for a resin spring, comprising the steps of forming an elastomer preform having an inner closed chamber; inserting an inner-pressure generating member into the inner closed chamber of the elastomer preform; and compressing the elastomer preform with the inner-pressure generating member by applying an external force to produce a permanently deformed portion having an oriented molecular structure.
After inserting the inner-pressure generating member into the inner closed chamber of the elastomer preform, an external force is applied to the elastomer preform and the inner-pressure generating member in the compressing step to produce permanent deformation and orient the molecular structure of the permanently deformed portion, thus carrying out an anti-setting treatment. Accordingly, by preliminarily making close contact of the inner-pressure generating member with the preform in the inserting step, the inner-pressure generating member is also deformed by deformation of the inner closed chamber of the preform by the external force applied in the compressing step, thereby highly ensuring close contact between the spring body and the inner-pressure generating member. As a result, the slip between the spring body and the inner-pressure generating member can be prevented regardless of repeated deformation in use, thereby obtaining stable load characteristics.
According to a second aspect of the present invention, the inserting step in the manufacturing method for the resin spring of the first aspect of the present invention also comprises injecting an inner-pressure generating resin into the inner closed chamber of the elastomer preform.
The inner-pressure generating resin is injected into the inner closed chamber of the elastomer preform to fill the inner closed chamber, so that the inner-pressure generating member molded from the inner-pressure generating resin is kept in close contact with the preform. Accordingly, close contact between the preform and the inner-pressure generating member is also highly maintained in the compressing step for carrying out the anti-setting treatment. As a result, the slip between the spring body and the inner-pressure generating member can be prevented regardless of repeated deformation in use, thereby obtaining stable load characteristics.
According to a third aspect of the present invention, there is provided a manufacturing method for a resin spring, comprising the steps of integrally forming an elastomer preform and an inner-pressure generating member; and compressing the elastomer preform with the inner-pressure generating member by applying an external force to produce a permanently deformed portion having an oriented molecular structure.
Because the elastomer preform and the inner-pressure generating member are integrally formed with each other, the inner-pressure generating member is in close contact with the elastomer preform prior to the compressing step.
Accordingly, the close contact between the preform and the inner-pressure generating member is also highly maintained in the compressing step. As a result, the slip between the spring body and the inner-pressure generating member can be prevented regardless of repeated deformation in use, thereby obtaining stable load characteristics.
According to a fourth aspect of the present invention, the integrally forming step in the manufacturing method for the resin spring of the third aspect of the present invention also comprises integrally forming the elastomer preform and the inner-pressure generating member so as to exhibit a slope function.
The integral body of the preform and the inner-pressure generating member have a slope function, so that the resin spring obtained by compressing the integral body does not have an interface where properties rapidly change. Therefore, the generation of cracks or the like in the resin spring due to stress concentration or the like are suppressed.
According to a fifth aspect of the present invention, there is provided a manufacturing method for a resin spring, comprising the steps of forming an elastomer preform having an inner closed chamber; compressing the elastomer preform by applying an external force to produce a permanently deformed portion whose molecular structure is oriented, thereby forming a spring body having the inner closed chamber deformed; and injecting an inner-pressure generating resin into the inner closed chamber of the spring body and molding the inner-pressure generating resin into an inner-pressure generating member by using the spring body as a part of a die.
The inner-pressure generating resin is injected into the inner closed chamber of the spring body obtained by the compressing step and is molded in this chamber by using the spring body as a part of the die. Accordingly, close contact between the spring body and the inner-pressure generating member can be highly ensured.
As a result, the slip between the spring body and the inner-pressure generating member can be prevented regardless of repeated deformation in use, thereby obtaining stable load characteristics.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.